Ion channels are cellular proteins that regulate the flow of ions, including calcium, potassium, sodium and chloride into and out of cells. These channels are present in all human cells and affect such physiological processes as nerve transmission, muscle contraction, cellular secretion, regulation of heartbeat, dilation of arteries, release of insulin, and regulation of renal electrolyte transport. Among the ion channels, potassium ion channels are the most ubiquitous and diverse, being found in a variety of animal cells such as nervous, muscular, glandular, immune, reproductive, and epithelial tissue. These channels allow the flow of potassium in and/or out of the cell under certain conditions. For example, the outward flow of potassium ions upon opening of these channels makes the interior of the cell more negative, counteracting depolarizing voltages applied to the cell. These channels are regulated, e.g., by calcium sensitivity, voltage-gating, second messengers, extracellular ligands, and ATP-sensitivity.
Potassium ion channels are typically formed by four alpha subunits, and can be homomeric (made of identical alpha subunits) or heteromeric (made of two or more distinct types of alpha subunits). In addition, certain potassium ion channels (those made from Kv, KQT and Slo or BK subunits) have often been found to contain additional, structurally distinct auxiliary, or beta subunits. These subunits do not form potassium ion channels themselves, but instead they act as auxiliary subunits to modify the functional properties of channels formed by alpha subunits. For example, the Kv beta subunits are cytoplasmic and are known to increase the surface expression of Kv channels and/or modify inactivation kinetics of the channel (Heinemann et al., J. Physiol. 493: 625-633 (1996); Shi et al., Neuron 16(4): 843-852 (1996)). In another example, the KQT family beta subunit, minK, primarily changes activation kinetics (Sanguinetti et al., Nature 384: 80-83 (1996)).
The alpha subunits of potassium ion channels fall into at least 8 families, based on predicted structural and functional similarities (Wei et al., Neuropharmacology 35(7): 805-829 (1997)). Three of these families (Kv, eag-related, and KQT) share a common motif of six transmembrane domains and are primarily gated by voltage. Two other families, CNG and SK/IK, also contain this motif but are gated by cyclic nucleotides and calcium, respectively. Small (SK) and intermediate (IK) conductance calcium-activated potassium ion channels possess unit conductances of 2-20 and 20-85 pS, respectively, and are more sensitive to calcium than are BK channels discussed below. For a review of calcium-activated potassium channels see Latorre et al., Ann. Rev. Phys. 51: 385-399 (1989).
Three other families of potassium channel alpha subunits have distinct patterns of transmembrane domains. Slo or BK family potassium channels have seven transmembrane domains (Meera et al., Proc. Natl. Acad. Sci. U.S.A. 94(25): 14066-14071 (1997)) and are gated by both voltage and calcium or pH (Schreiber et al., J. Biol. Chem. 273: 3509-3516 (1998)). Slo or BK potassium ion channels are large conductance potassium ion channels found in a wide variety of tissues, both in the central nervous system and periphery. These channels are gated by the concerted actions of internal calcium ions and membrane potential, and have a unit conductance between 100 and 220 pS. They play a key role in the regulation of processes such as neuronal integration, muscular contraction and hormone secretion. They may also be involved in processes such as lymphocyte differentiation and cell proliferation, spermatocyte differentiation and sperm motility. Members of the BK (Atkinson et al., Science 253: 551-555 (1991); Adelman et al., Neuron 9: 209-216 (1992); Butler, Science 261: 221-224 (1993)) subfamily have been cloned and expressed in heterologous cell types where they recapitulate the fundamental properties of their native counterparts. Finally, the inward rectifier potassium channels (Kir), belong to a structural family containing two transmembrane domains, and an eighth functionally diverse family (TP, or “two-pore”) contains two tandem repeats of this inward rectifier motif.
Each type of potassium ion channel shows a distinct pharmacological profile. These classes are widely expressed, and their activity hyperpolarizes the membrane potential. Potassium ion channels have been associated with a number of physiological processes, including regulation of heartbeat, dilation of arteries, release of insulin, excitability of nerve cells, and regulation of renal electrolyte transport. Moreover, studies have indicated that potassium ion channels are a therapeutic target in the treatment of a number of diseases including central or peripheral nervous system disorders (e.g., migraine, ataxia, Parkinson's disease, bipolar disorders, trigeminal neuralgia, spasticity, mood disorders, brain tumors, psychotic disorders, myokymia, seizures, epilepsy, hearing and vision loss, psychosis, anxiety, depression, dementia, memory and attention deficits, Alzheimer's disease, age-related memory loss, learning deficiencies, anxiety, traumatic brain injury, dysmenorrhea, narcolepsy and motor neuron diseases), as well as targets for neuroprotective agents (e.g., to prevent stroke and the like); as well as disease states such as gastroesophogeal reflux disorder and gastrointestinal hypomotility disorders, irritable bowel syndrome, secretory diarrhea, asthma, cystic fibrosis, chronic obstructive pulmonary disease and rhinorrhea, convulsions, vascular spasms, coronary artery spasms, renal disorders, polycystic kidney disease, bladder spasms, urinary incontinence, bladder outflow obstruction, ischemia, cerebral ischemia, ischemic heart disease, angina pectoris, coronary heart disease, Reynaud's disease, intermittent claudication, Sjorgren's syndrome, arrhythmia, hypertension, myotonic muscle dystrophia, xerostomia, diabetes type II, hyperinsulinemia, premature labor, baldness, cancer, and immune suppression.
Specifically, SK channels have been shown to have distinct pharmacological profiles. For example, using patch clamp techniques, the effects of eight clinically relevant psychoactive compounds on SK2 subtype channels were investigated (Dreixler et al., Eur. J. Pharmacol. 401: 1-7 (2000)). The evaluated compounds are structurally related to tricyclic antidepressants and include amitriptyline, carbamazepine, chlorpromazine, cyproheptadine, imipramine, tacrine and trifluperazine. Each of the compounds tested was found to block SK2 channel currents with micromolar affinity. A number of neuromuscular inhibiting agents exist that affect SK channels, e.g. apamin, atracurium, pancuronium and tubocurarine (Shah et al., Br J Pharmacol 129: 627-30 (2000)).
Moreover, patch clamp techniques have also been used to study the effect of the centrally acting muscle relaxant chlorzoxazone and three structurally related compounds, 1-ethyl-2-benzimidazolinone (1-EBIO), zoxazolamine, and 1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one (NS 1619) on recombinant rat brain SK2 channels (rSK2 channels) expressed in HEK293 mammalian cells (Cao et al., J. Pharmacol. Exp. Ther. 296: 683-689 (2001)). When applied externally, chlorzoxazone, 1-EBIO, and zoxazolamine activated rSK2 channel currents in cells dialyzed with a nominally calcium-free intracellular solution.
The effects of metal cations on the activation of recombinant human SK4 (also known as hIK1 or hKCa4) channels has also been studied (Cao and Houamed, FEBS Lett. 446: 137-41 (1999)). The ion channels were expressed in HEK 293 cells and tested using patch clamp recording. Of the nine metals tested, cobalt, iron, magnesium, and zinc did not activate the SK4 channels when applied to the inside of SK4 channel-expressing membrane patches. Barium, cadmium, calcium, lead, and strontium activated SK4 channels in a concentration-dependent manner. Calcium was the most potent metal, followed by lead, cadmium, strontium, and barium.
The SK channels are heteromeric complexes that comprise pore-forming α-subunits and the calcium binding protein calmodulin (CaM). CaM binds to the SK channel through the CaM-binding domain (CaMBD), which is located in an intracellular region of an α-subunit close to the pore. Based on a recently published crystal structure, calcium binding to the N-lobe of the CaM proteins on each of the four subunits initiates a structural change that allows a hydrophobic portion of the CaM protein to interact with a CaMBD on an adjacent subunit. As each N-lobe on an adjacent subunit grabs the other CaMBD C-terminal region, a rotary force is thought to be created between them which would drive open the channel.
New classes of compounds that act to modulate the opening of potassium ion channels would represent a significant advance in the art and provide the opportunity to develop treatment modalities for numerous diseases associated with these channels. The present invention provides a new class of potassium ion channel modulators and methods of using the modulators.